Blocks: Use do_blocks to render core blocks content

[aduth]

Fixes #6662

This pull request seeks to resolve an issue where shared blocks which contain InnerBlocks content (e.g. Columns) would not render its content on the front-end.

Implementation notes:

The root cause is that we were parsing the content as blocks and relying on the parsed block's innerHTML to be the render output. For nested blocks, the innerHTML does not include its child block contents. It is also not possible to reconstruct this block on the server.

Instead, the approach here simply treats the shared block's content as if it were a post containing blocks, leveraging the do_blocks function to pass its content string. This should support not only inner blocks, but also dynamic block replacement (e.g. shared blocks in shared blocks).

This may also have a performance benefit, avoiding a costly full content parse in lieu of do_blocks simpler implementation.

Testing instructions:

Repeat steps to reproduce from #6662, noting there are no issues in the display of the shared Columns block on the front-end of the site.

[tofumatt]

Tested locally and seems to work for me!

I think this is worth landing now but e2e tests for this would be good in the future.

[aduth]

I think this is worth landing now but e2e tests for this would be good in the future.

Agree! I created an issue at #8054 to track.